U.S. Pat. No. 6,797,956 discloses a phase contrast electron microscope having phase contrast imaging. By utilizing an annularly-shaped illuminating aperture, no small components are needed for the phase-shifting element which are critical with respect to electrostatic charging and contamination compared to other phase contrast systems. However, more detailed analysis has shown that the phase shift in this known system is achieved essentially by the intense spherical aberration of the einzel lens in the peripheral region.
Additional systems for generating phase contrast in a transmission electron microscope are disclosed in United States patent application publications 2008/0296509 and 2007/0284528 and U.S. Pat. Nos. 5,814,815 and 6,797,956.